An actuating device of the type in question for a convertible top of a convertible is known from DE 195 18 071 A1. The actuating device described there is actuated hydraulically. The convertible top itself is fastened there on the left and right sides thereof to respective gearing arrangements which are composed of a multiplicity of links coupled mechanically to one another. The two gearing arrangements here have the task of mechanically guiding the convertible top during the adjustment between the closed position and the open position. For a satisfactory functioning of the convertible top, a synchronous running between the left gearing arrangement and the right gearing arrangement is imperative.
In order to drive the two gearing arrangements, DE 195 18 071 A1 describes two hydraulic drive cylinders which each act on one of the two gearing arrangements. This means, in other words, that the hydraulic adjustment of the two drive cylinders is transmitted via the extension and the retraction of the two piston rods to the gearing arrangements in order thereby to open or to close the convertible top. The two drive cylinders of the actuating device described in this document are connected into triangles. This means, in other words, that the annular volume of the first drive cylinder, the size of which is determined by the inside diameter of the drive cylinder and by the outside diameter of the piston rod, is hydraulically connected to the circular volume of the second drive cylinder, which is determined by the inside diameter of the drive cylinder, via a connecting line. Via said connecting line, the hydraulic drive fluid is pressed out of the annular volume of the first drive cylinder into the circular volume of the second drive cylinder, or, conversely, out of the circular volume of the second drive cylinder into the annular volume of the first drive cylinder. This hydraulic series connection has the advantage that the two drive cylinders can be hydraulically driven by the fact that the one drive cylinder is adjusted by a hydraulic drive device, for example a hydraulic pump. The adjustment movement of the one drive cylinder is then transmitted by displacement of the hydraulic drive fluid out of the annular volume of the first drive cylinder or out of the circular volume of the second drive cylinder to the drive cylinder downstream in each case via the connecting line. In this way, only one of the two drive cylinders has to be connected directly to the hydraulic drive source.
If the two drive cylinders are connected hydraulically in series, the drive fluid from the annular volume or from the circular volume of the upstream drive cylinder is transferred to the circular volume or annular volume of the downstream drive cylinder. The displaced volumetric flow from the upstream drive cylinder therefore corresponds to the pumped-in volume of the downstream drive cylinder. However, it should be noted here that, when the inside diameter of the drive cylinders is identical or when the outside diameter of the piston rods is identical, said constant volumetric flow between the two drive cylinders results in the two piston rods being extended at different speeds. This is because the annular volume of a drive cylinder is in each case smaller than the circular volume because of the diameter of the piston rod reaching through the annular volume. If, for example, a drive cylinder is then connected in series to a second drive cylinder in such a manner that the hydraulic oil flows out of the annular volume of the first drive cylinder into the circular volume of the second drive cylinder via the connecting line, then, when the drive cylinders are structurally identical in each case, the piston rod of the second drive cylinder is extended more slowly because of the difference in volume between the annular volume of the second drive cylinder and the circular volume of the first drive cylinder.
If, for a convertible top system, synchronous running between the first gearing arrangement and the second gearing arrangement is required, it is known from the prior art that the annular volume of the first drive cylinder and the circular volume of the second drive cylinder, said volumes each being connected hydraulically to each other via a connecting line, have to be suitably coordinated with each other. Said coordination consists in that the annular area of the first drive cylinder, said annular area forming the annular volume, and the circular area of the second drive cylinder, said circular area forming the circular volume, are precisely the same size. The effect achieved by said coordination in terms of area of the annular area of the first drive cylinder, which is produced from the difference between the internal cross section of the drive cylinder and the external cross section of the piston rod, and the circular area of the second drive cylinder, which is produced from the internal cross section of the second drive cylinder, is that the two drive cylinders can move synchronously with respect to each other despite the hydraulic series connection therebetween.
A disadvantage of this type of series connection, which is known from the prior art, of two drive cylinders of an adjustable convertible top system is that the choice of possible drive cylinders for realizing the convertible top system is considerably limited. This is because, when standard drive cylinders are used, it is customarily not possible to ensure, by choosing different constructional sizes for the two drive cylinders, that the annular area of the first drive cylinder will correspond to the circular area of the second drive cylinder. For the realization of adjustable convertible top systems with drive cylinders connected in series, the corresponding drive cylinders have therefore previously had to be produced as custom-manufactured articles corresponding to the required dimensioning. If, for example, the first drive cylinder was selected as a standard hydraulic cylinder, the second drive cylinder would have had to be precisely coordinated with the overall dimensions of the first drive cylinder, which was possible only by correspondingly expensive individual custom-manufactured articles.